1. Field of the Invention
The present invention relates to ink jet printers, and, more particularly, to a method of printing with an ink jet printer using an enhanced horizontal resolution.
2. Description of the Related Art
An ink jet printer includes a printhead having a plurality of ink emitting orifices therein (FIG. 1). For a multi-color ink jet printer, the ink emitting orifices in the printhead are typically segregated into separate arrays of ink emitting orifices, with each array corresponding to a different color ink to be jetted onto a print medium. A plurality of ink jetting heaters which are associated with respective ink emitting orifices are selectively actuated to jet the ink from the ink emitting orifices, in known manner.
The printhead is typically mounted on and carried by a movable carriage assembly. The print medium is movable in an advance direction within the printer, and the carriage assembly is movable in a transverse or orthogonal direction relative to the advance direction. An image area is defined which overlies the print medium. The image area includes a plurality of rows and columns of pixel locations. Each separate pixel location corresponds to an ink dot placement location on the print medium. The resolution or center-to-center spacing between the pixel locations is typically predefined in both the vertical (i.e., advance) direction and the horizontal (i.e., transverse) direction. The vertical resolution of the pixel locations is defined by the center to-center spacing between the ink emitting orifices. The horizontal resolution of the pixel locations is defined by frequency response limitations associated with the printhead. For example, the ink feed channel and nozzle section associated with each ink emitting orifice require a certain amount of time to refill after the associated ink jetting heater is actuated during printing. The transverse resolution of the pixel locations may be a function of the ink refill time for the orifices in the printhead. A typical printhead used in an ink jet printer places ink dots in an image area on a print medium at an advance resolution of 300 dpi and a transverse resolution of 300 dpi.
Notwithstanding the frequency response limitations of a printhead which may define the actual horizontal resolution of the pixel locations in an image area, the printhead may actually have an addressable resolution in the transverse direction which is greater than the actual resolution of the pixel locations in the transverse direction. More particularly, the carriage assembly which carries the printhead is typically driven by a stepper motor which steps the carriage assembly across the print medium in the transverse directions in incremental steps. The stepper motor has a minimum step distance which corresponds to the addressable resolution of the printhead in the transverse direction. For example, the actual transverse resolution of the printhead may be set at 300 dots per inch (dpi) because of frequency response limitations associated with the printhead, while the addressable resolution of the printhead in the transverse direction may correspond to 600 dpi. With such an example, the addressable resolution of the printhead is actually twice that of the actual transverse resolution of the printhead.
It is known to provide an enhanced print quality in a monochrome (i.e., single color) ink jet printer by taking advantage of the addressable transverse resolution of the printhead. For example, U.S. Pat. No. 5,480,240, assigned to the assignee of the present invention, discloses a method of providing enhanced print quality by enhancing the resolution of the print image along the horizontal axis. Print data which is stored at a lower resolution grid format is shifted to a finer resolution grid format associated with the addressability of the printhead in the horizontal direction. The center of the ink dot placement locations and the size of the ink dots may be varied using the finer resolution grid to provide the enhanced print quality.
Printing with a multi-color ink jet printer typically utilizes a halftoning process to determine the relative frequency of ink dots and their arrangement on the print image to produce colors not in the set of primary colors. A threshold matrix of a predetermined size includes a plurality of rows and columns of cells, with each cell corresponding to a pixel location in the image area. Each cell is assigned a threshold value which is scaled proportionately to normalize the colors to the size of the matrix used. As the print image is analyzed, the position of a cell within the threshold matrix is used to determine the threshold value for that particular cell. Each of the primary colors are assigned a color value. If the color value for the primary color is greater than the threshold value within a cell of the threshold matrix, ink is jetted at a corresponding pixel location in the image area.
Referring to FIG. 2, an example of a threshold matrix for use during a halftoning process during color printing is shown. The threshold matrix includes four columns and four rows, with a total of sixteen cells. Each cell corresponds to one of the pixel locations in the image area and is assigned a threshold value as indicated. The primary color cyan is assigned a color value of 8; magenta is assigned a color value of 4; and yellow is assigned a color value of 12. Cyan ink would thus be printed at pixel locations corresponding to the substantially filled cells indicated by the cyan print plane. It should be noted that the color values assigned to each primary color can generally be thought of as corresponding to a percentage of that particular primary color ink to be jetted onto the print medium within a portion of the image area corresponding to the threshold matrix. That is, for the example shown, the threshold matrix includes sixteen total cells and cyan ink is assigned a color value eight. Thus, approximately eight out of sixteen cells (i.e., approximately 50%) of the cells are assigned a cyan color value. This correspondingly means that approximately 50% of the pixel locations corresponding to the threshold matrix cells likewise receive cyan ink. Likewise, the yellow ink would be placed at approximately 12/16 or 75% of the pixel locations for the threshold matrix on the corresponding portion of the image area. The threshold matrix can thus be thought of as a frequency distribution of the various color ink dots within a predefined and corresponding portion of the image area.
What is needed in the art is a method of providing an enhanced resolution along the horizontal axis with a color ink jet printer, without affecting adversely a halftoning printing technique.